Moldable pellet based on the combination of synthetic cellulose fibers and thermoplastic polymers

ABSTRACT

A moldable pellet used for making high impact, non-abrasive recyclable structural composites consisting of a thermoplastic polymer or polymers, with or without fillers and additives, and a synthetic cellulosic fiber in yarn or tow form such as Rayon or Lyocell. The concentration of cellulose fiber within the pellet may vary from approximately 2-80 percent by weight or higher. This moldable pellet is suitable for molding in current molding applications such as, but not limited to, injection molding, extrusion compression molding, and compression molding.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

The present invention relates generally to structural composites andmore specifically to moldable pellets consisting of a combination ofthermoplastic polymers and synthetic cellulosic fibers that may be madeinto structural composites.

BACKGROUND OF THE INVENTION

Structural composites are well known and are used in a wide variety ofapplications that require plastic parts having a minimum threshold ofmechanical properties such as strength and impact resistance whileimparting cost savings in terms of manufacturing techniques and in termsof weight reduction. Examples of structural composites include sheetmolding compound (SMC), fiber reinforced thermoplastics and structuralreinforced injection molding (SRIM).

Introducing glass fiber or other reinforcing material into athermoplastic or thermosetting polymer material typically makesstructural composites. The glass fiber and polymer material may be mixedtogether and formed into a composite part in a wide variety of methods,including compression molding and injection molding. Structuralcomposites made of glass fiber or other reinforcing material offergenerally good mechanical properties in terms of impact, toughness,strength and may be used in a wide variety of applications.

One problem with glass reinforced or carbon fiber reinforced compositesis that the reinforcement fibers are generally abrasive. Thisabrasiveness can adversely affect equipment used to mold the compositeparts. This in turn increases the cost for manufacturing reinforcedcomposites parts due to increased mold turnover and downtime associatedwith mold turnover.

Another problem with glass reinforced or carbon fiber reinforcedcomposites is that the fiber tends to break during injection molding andextrusion compression molding processing. Thus, recycled parts made ofreinforced composites lose significant mechanical properties associatedwith fiber length within the composite material during processing.Impact resistance is, in most cases, the most significantly affectedmechanical property. However, strength and modulus may suffer as well.

Further, composite parts cannot be recycled without further degradingfibers within the composite material. Therefore, composite parts notmade to exact specifications are disposed of as waste.

SUMMARY OF THE INVENTION

It is thus an object of the present invention to reduce abrasiveness infiber reinforced structural composites without adversely affectingmechanical properties of the finished structural part.

It is another object of the present invention to process and recyclecomposite components without significantly reducing fiber length orcompromising mechanical performance.

It is a further object of the present invention to reduce complexity informing composite parts associated with traditional injection moldingand compression molding techniques.

The above objects are accomplished by producing a moldable pelletconsisting of a thermoplastic polymer or polymers, with or withoutfillers and additives, and a synthetic cellulosic fiber such as Rayon orLyocell. The concentration of cellulose fiber within the pellet may varyfrom approximately 2-38 percent by weight or higher. This moldablepellet is suitable for molding in current molding applications such as,but not limited to, injection molding and extrusion compression molding.

It has been discovered that the impact performance of thermoplasticsreinforced with synthetic cellulose fibers is excellent, typicallysuperior to glass, carbon, natural fiber, or talc-reinforcedthermoplastics and competitive with several impact resistant polymerssuch as ABS (acrylonitrile-butadiene-styrene), PC (polycarbonate)-ABS,Dylark, and other high impact polymers. In addition, synthetic fibersare non-abrasive and therefore will produce minimal wear on moldingequipment. Also, because synthetic fibers are inherently tough,composite components may be processed and recycled without significantlyreducing fiber length or compromising mechanical performance.

Other objects and advantages of the present invention will becomeapparent upon considering the following detailed description andappended claims, and upon reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 describes a process for forming a moldable pellet according to apreferred embodiment of the present invention;

FIG. 2 is a section view of a moldable pellet made according to FIG. 1;

FIG. 3 is a graphical comparison of notched IZOD impact properties ofvarious structural composites at varying fiber levels;

FIG. 4 is a graphical comparison the effect of oil sizing on the notchedIZOD impact properties of a structural composite made with the moldablecomposite of the present invention; and

FIG. 5 is a graphical comparison the effect of oil sizing on theunnotched IZOD impact properties of a structural composite made with themoldable composite of the present invention.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

Referring now to FIG. 1, a schematic diagram for making a moldablepellet 22 is generally designated as 10.

The pellets 22 are produced by first passing a cellulosic yarn or tow 12of between approximately 0.87 g/m-4.37 g/m (0.8-4.0 g/yard) through acone-shaped dye 14, or wire coater, designed to evenly spread athermoplastic polymer 16 circumferentially around the travelling fiberyarn or tow 12. For the purposes of clarity, yarn or tow describes thecoupling of individual fibers that are well known to a person skilled inthe art. For ease of description, however, yarn or tow are usedinterchangably within this application. Thus, where yarn alone isdescribed, it is understood that a tow could also be used in place ofthe yarn. Further, where the phrase “yarn or tow” is used, it should beunderstood that either a fiber yarn or fiber tow may be used.

The encapsulated yarn 12 may then be passed through a water bath 18immediately to solidify the polymer sheath or may be air-cooled. Theencapsulated yarn 12 is then chopped or pelletized using commerciallyavailable choppers 20 and pelletizers (not shown) in varying lengths.The pellets 22 are chopped to lengths ranging from approximately 3mm-600 mm (⅛ inch to 24 inches) or more in length, depending upon theapplication. Preferably, a Conair Jetro Pelletizer Model #2047 or anOwens Corning CB-83 chopper are used, with the latter being morepreferable for mass production and quality control.

The chopped or pelletized composite fibers 22 are then fed into amolding machine 24 and molded into composite parts 26. Preferably, themolding machine 24 is an injection molding machine or an extrusioncompression molding machine. However, other compression molding machinesmay also be used as are known in the art. The temperature within themolding machine 24 during the molding process is kept betweenapproximately 170-240° C. (340-460° Fahrenheit) to ensure proper meltingand flow of the thermoplastic polymer 16 without degrading thecellulosic fiber yarn 12. Preferably, the parts 26 are molded attemperatures less than 240° C. (460° F.) to avoid thermal degradation ofthe cellulose fibers in the yarn 12. The concentration of cellulosefiber yarn 12 within the pellet may vary from approximately 2-38 percentby weight or higher, with higher percentages resulting in higher impactresistance composite parts 26. A comparison of impact resistantproperties of cellulosic fibers and other fibers is described below inFIG. 3.

As best seen in FIG. 2, the moldable pellet 22 that is formed consistsof the cellulosic fiber yarn 12 encapsulated within a thermoplasticpolymer 16. The cellulosic fiber yarn 12 is preferably either Rayon orLyocell and comprises between approximately 2 and 38 percent or more ofthe total weight of the moldable pellet 22 which makes up the compositepart 26 when using an injection molding process or extrusion compressionmolding process. However, the cellulosic fiber yarn 12 preferablycomprises approximately 50 to 80 weight percent when using othercompression molding processes. Preferably, the thermoplastic polymer 16comprises a high melt flow index polypropylene polymer homopolymer orcopolymer. However, other thermoplastic polymers 16 may be used as longas the processing temperature remains below the temperature at whichdegradation of the cellulosic fiber yarn 12 occurs, which is around 230°C. (450° F.). Thus, certain nylons, polycarbonates, andacrylonitrile-butadiene-styrene (“ABS”) polymers may be used as thethermoplastic polymer 16.

In addition, moldable pellets 22 may be dry-mixed with other polymers toenhance resin matrix properties. For example, the pellets 22 could bedrymixed with low melt flow index polypropylenes and polyethylenes. Inaddition, pigmented resins may be added to the thermoplastic polymer 16masterbatch to produce colored composite parts 26. Again, as above, theyarn 12 comprises approximately 2 to 38 percent by weight of the totalweight of the composite part 26. This masterbatching operation providesan easy method to control fiber concentration, pigmentation, andproperties of the molded composite part 26.

To aid in dispersing the yarn 12 fibers in the thermoplastic resin 16when using an injection molding process or extrusion compression moldingprocess, a size is preferably introduced to the cellulose yarn 12 fiberprior to the wire-coating step. The size is used to aid in dispersingthe yarn 12 fibers within the matrix resin 16 during the melting/mixingstep that occurs in the screw (extruder) section of a typical injectionmolding machine prior to injection into the molding chamber. This sizemay be an oil, lubricant, wax, polymeric, or non-polymeric material thatis known in the art and applied in a wide variety of techniques,including by dipping, spraying or through the use of a pump. If an oilsize is used, good fiber dispersions are obtained in thermoplasticresins 16 with exceptionally high melt flow indices (on the order of500) when a 10-20% mineral oil sizing is applied to the cellulosic fiberyarn 12.

Alternatively, a peroxide wax additive may be used in place of a sizefor injection molding purposes. The peroxide wax is used to reduce theviscosity of the thermoplastic polymer 16 to enable dispersion of theyarn 12 in the injection molding and extrusion compression moldingprocess. Also, the peroxide wax may be incorporated into the wirecoating process to reduce the viscosity of the wire coating resin. Anexample of a preferred formulation for a pellet 22 chopped to 6-13 mm(¼-½ inch) chop containing the peroxide wax includes rayon fiber (60% byweight), polypropylene having a melt flow index of 35 (37% by weight), amaleated polypropylene such as Polybond 3200 (2% by weight), and theperoxide wax (1% by weight). Additional polypropylene is dry mixed withthe pellet to reduce the fiber concentration in the composite part 26 toapproximately 33% by weight.

Of course, when a compression molding machine is used as the moldingmachine 24, this size is not needed, as there is no premixing necessaryto form the composite part 26. In this process, pellets 22 of differentchop lengths, from 6-13 mm (¼-24 inches) in length or more, are addeddirectly to the compression molding equipment without apre-melting/mixing step. The thermoplastic resin 16 in the pellet 22melts during the compression molding process and impregnates the spacessurrounding the fiber 12 yarn, therein forming the composite part 26.

The effect of the sizing and temperature on notched and unnotched IZODimpact properties is described below in FIGS. 3, 4 and 5.

Referring now to FIG. 3, a comparison of notched IZOD properties forvarious reinforced polypropylene composite parts made in accordance withthe preferred embodiment of FIG. 1 and the thermoplastic resincomposition of FIG. 2 is illustrated. Here, rayon was used as thereinforcing cellulosic yarn 12 and the pellet 22 was chopped toapproximately 13 mm (½ inch) in length. As seen in FIG. 3, compositesreinforced with rayon according to a preferred embodiment of the presentinvention, as shown on line 100, displayed improved notched IZOD impactresistance, particularly above 25% weight percent fiber, as comparedwith other natural and glass reinforced composites. These other naturaland glass reinforced composites include dry use chopped strands(“DUCS”), as seen on line 105, talc filled polypropylene, as seen online 110, natural fiber composites, as seen on line 120, and glassbundle sheaths covered with polypropylene, such as Owens Corning Stamax,as seen on line 130.

Referring now to FIGS. 4 and 5, the effect of oil sizing on the notchedand unnotched IZOD properties of Rayon-polypropylene extruded compositesat room temperature and −40 degrees Celsius is illustrated. As seen inFIG. 4, the oil sizing improved the notched IZOD properties and arayon-polypropylene composite at room temperature and −40 degreesCelsius with 10 and 20 percent rayon loading. For Unnotched IZODproperties, as seen in FIG. 5, the oil sizing appeared to have a slightdetrimental effect, especially at higher loadings of twenty ortwenty-five percent.

It has been discovered that the impact performance of thermoplasticsreinforced with synthetic cellulose fibers is excellent, typicallysuperior to glass, carbon, natural fiber, or talc-reinforcedthermoplastics and competitive with several impact resistant polymerssuch as ABS (acrylonitrile-butadiene-styrene), PC(polycarbonate)-ABS,Dylark, and other high impact polymers.

In addition, synthetic fibers are non-abrasive and therefore willproduce minimal wear on molding equipment. This in turn decrease costsin terms of mold turnover and downtime associated with mold turnover.

Also, because synthetic cellulosic fibers are inherently tough,composite components may be processed and recycled without significantlyreducing fiber length or compromising mechanical performance.

Finally, the moldable pellet simplifies injection molding andcompression molding techniques and improves part quality associated withthese techniques by allowing a more uniform dispersion of fiber withinthe composite part.

While the invention has been described in terms of preferredembodiments, it will be understood, of course, that the invention is notlimited thereto since modifications may be made by those skilled in theart, particularly in light of the foregoing teachings.

What is claimed is:
 1. A moldable pellet used for making high impact, low abrasive recyclable structural composites comprising: an inner cellulosic fiber yarn; and a thermoplastic polymer sheath surrounding said inner cellulosic fiber yarn.
 2. The moldable pellet of claim 1, wherein said inner cellulosic fiber yarn comprises between approximately 2 and 80 percent by weight of the moldable pellet.
 3. The moldable pellet of claim 1, wherein said thermoplastic polymer sheath is selected from the group consisting of a high melt flow index polypropylene polymer sheath, a high melt flow index polyethylene polymer sheath, a high melt flow index nylon polymer sheath, a high melt flow index polycarbonate polymer sheath, and a high melt flow index ABS polymer sheath.
 4. The moldable pellet of claim 1, wherein said inner cellulosic fiber yarn comprises a Rayon fiber yarn.
 5. The moldable pellet of claim 1, wherein said inner cellulosic fiber yarn comprises a Lyocell fiber yarn.
 6. The moldable pellet of claim 1, wherein the weight of said said inner cellulosic fiber yarn is between approximately 0.8 and 4.0 grams/yard.
 7. The moldable pellet of claim 1, wherein the length of said moldable pellet is between approximately one-eighth inch and twenty-four inches after chopping or pelletizing.
 8. The moldable pellet of claim 1 further comprising a low viscosity sizing composition coupled to said inner cellulosic fiber yarn and within said thermoplastic polymer sheath.
 9. The moldable pellet of claim 8, wherein said low viscosity sizing composition is selected from the group consisting of an oil sizing composition, a lubricant, a wax, a polymeric sizing composition, and a non-polymeric sizing composition. 